Low carbon sheet steels find wide usage in the automotive industry due, at least in part, to their ability to be formed by stamping at relatively high production rates. Substantial material and weight savings can be accomplished if the total elongation, i.e., the amount that a steel sheet can be strained in a tensile test before fracture, is improved. A quantitative measure of the relative formability of different sheet steels is not readily determinable by laboratory tests. However, total elongation, as measured in a tensile test, does show correlation with formability. For example, if like gage steel sheet tensile specimen are tested, that exhibiting the greatest total elongation will have the greatest formability. Increased formability, evidenced by the ability to form deeper draws from like gage steel, allows greater latitude in part design. For example, it may obviate the need to form a part having a deep cross section in two pieces and later weld them together.
Before this invention, the formability of low carbon sheet steel was improved by raising the temperature of the sheet substantially above room temperature during forming. Another approach has been to vacuum de-gas sheet steel prior to forming to eliminate interstitial elements such as carbon, nitrogen, and oxygen and produce a substantially interstitial free steel. The absence of interstitial elements improves ductility. However, this process is relatively expensive, time consuming, and the interstitial free steel is not as strong.